My invention relates to a received signal strength indicator for a radio receiver, and particularly to such an indicator that is temperature compensated.
In radio receivers, particularly frequency modulation (FM) receivers, it is often convenient or desirable to have a received signal strength indication (RSSI). A received signal strength indicator provides a very effective method of tuning a radio receiver, and it can be utilized as part of a carrier squelch circuit. A received signal strength indicator is mandatory or a very necessary part of radio telephone cellular systems which monitor the strengths of received signals, and allocate specific channels for utilization on the basis of those signal strengths.
In FIG. 1, I show a block diagram of a prior art radio receiver having a received signal strength indicator. Such a receiver includes an antenna 10 which supplies radio signals to one or more stages of RF amplifiers 11. The amplified radio signals are applied to a mixer 12 along with a suitable signal from an oscillator 13 to produce an intermediate frequency (IF) signal. This IF signal is filtered in a filter 14. The filtered IF signal is then amplified by one or more stages of IF amplifiers. As an example, I have assumed that the receiver utilizes two intermediate frequency (IF) amplifiers 15, 16 coupled in cascade. More or less intermediate frequency amplifiers may be provided. The signal output from the second IF amplifier 16 is applied to a discriminator 17. The output of the discriminator 17 may be suitably amplified and applied to an output device such as a loudspeaker. In FIG. 1, I have shown each of the IF amplifiers 15, 16 producing a received signal strength indication (RSSI). However, persons skilled in the art will appreciate that received signal strength indications may also be derived from the RF amplifier 11, if suitable filtering is provided.
FIG. 2 shows a schematic diagram of one of the IF amplifiers 15, 16 of FIG. 1, which typically are similar or identical. Such an amplifier includes a suitable source of direct current voltage B+ supplied to a bus 20 with respect to a ground or reference potential 21. The signal input from either the filter 14 or a preceding IF amplifier is applied to terminals 22, 23 which are respectively coupled to the base electrodes of similar NPN transistors Q1, Q2. The collectors of these transistors are coupled to the voltage bus 20 through respective similar resistors R4, R5. The emitters are coupled to a common junction 24 which in turn is coupled through a current source 25 to the ground 21. The base electrodes of the transistors Q1, Q2 are respectively coupled through similar resistors R1, R2 to the base electrode of an NPN transistor Q3. The emitter of the transistor Q3 is coupled through a resistor R3 to the junction 24, and the collector of the transistor Q3 provides the received signal strength indication (RSSI). This RSSI is coupled to a suitable voltage as will be explained. The amplified signal output is derived from the collectors of the transistors Q1, Q2, and may either be applied to another intermediate frequency amplifier or to the discriminator 17.
The circuit of FIG. 2 is known in the art. When a relatively large signal is applied to the input terminals 22, 23, the signal at the junction 24 is a full wave rectified signal. The transistor Q3 has its base electrode biased midway between the input signals at the terminals 22, 23, and hence remains at a constant potential regardless of signal level. Under small signal input conditions, the collector current of the transistor Q3 is determined by the magnitude of the resistor R3 and the direct current voltage at the junction 24. When the input signal level increases, the full wave rectified signal of the transistors Q1, Q2 tends to raise the emitter voltage of the transistor Q3, and reduce its collector current. Thus, the RSSI current to the collector of the transistor Q3 decreases as the input signal level increases, and conversely increases as the input signal level decreases. If the individual RSSI output currents (from each circuit like FIG. 2) are summed and filtered, a quasi logarithmic curve of RSSI output current in microamperes (.mu.A) versus signal input level in dbm is provided, as shown in FIG. 3.
It is known in the art that typical NPN type transistors such as transistors Q1, Q2 and Q3 are temperature sensitive. It can be shown for the circuit of FIG. 2 that as the ambient temperature increases, the RSSI current increases, and conversely as the ambient temperature decreases, the RSSI current decreases. Such sensitivity to changes in ambient temperature reduces the accuracy and performance of a radio receiver relying on RSSI such as shown in FIG. 1, so that temperature compensation is highly desirable, if not essential.